1. Field of the Invention
This invention relates to electronic devices, and particularly to devices incorporating the so-called "split gate" structure or its equivalent.
2. Description of Related Art
A known split-gate structure comprises a substrate of undoped gallium arsenide on which is formed a layer of aluminium gallium arsenide. Source and drain contacts are provided on the AlGaAs layer, and a metal Schottky electrode gate is also formed over part of that layer. The structure is therefore basically a device of the high-electron-mobility transistor (HEMT) type, in which the electron gas is confined to a very thin region of the GaAs layer adjacent the GaAs/AlGaAs interface. The electron transport is therefore virtually two-dimensional, and parallel to the interface. However, in the split gate device a very short and narrow (e.g. .ltoreq.1 .mu.m long and .ltoreq.1 .mu.m wide) gap is formed across the gate electrode, the gap extending in a direction from the source electrode to the drain electrode. When a negative gate bias voltage of sufficient magnitude is applied to the device, the electron path from the source to the drain is virtually cut off, and electrons can pass only under the gap. The electron transport is reduced to one dimension. The relevant electron states which carry the current are called sub-bands. The action is as follows. Electrons cannot occupy the same energy state and they therefore increase in energy with increasing number. The maximum value of electron momentum corresponds to the Fermi level or energy E.sub.f, which at low temperature (e.g. 77.degree. K.) is the highest occupied energy state. All sub-bands must be occupied up to the same total energy or the electrons will redistribute themselves to achieve that result. The higher sub-bands therefore contain fewer electrons.
As the negative gate bias voltage is increased, the width of the electron channel decreases. This increases the energy of the sub-bands. The sub-band with the maximum energy then becomes too high in energy to remain occupied, and the electrons which were in it pass into other levels. As the negative bias is further increased, the levels successively depopulate until the only electrons remaining are in the ground-state sub-band.
The flow of current through the gap is determined by the electron concentration and the electron velocity at the Fermi energy. Where there is no scattering, the change in kinetic energy of the electrons is determined by the source/drain voltage V.
It can be shown that the current J is given by EQU J=2ie.sup.2 V/h (1)
where i is the number of sub-bands present, e is the electron charge, h is Planck's constant and the factor 2 counts electron spin. From equation (1) it follows that the resistance R of the constricted path under the gap is given by ##EQU1##
The terms are all constants, and for a single sub-band the equation gives a resistance of 25.7 k.OMEGA., irrespective of the parameters of the material and the length of the gap. If more than one sub-band is present, the resistance is reduced accordingly. The ballistic resistor formed by the split gate structure does not follow the normal laws for connection of resistors in series or in parallel. If two equal split gates are located in series between the source and drain electrodes, the resistance will remain equal to that of a single split gate device. If, due to their having unequal numbers of sub-bands, the resistors are of unequal values, the total resistance of two such resistors in series will be equal to that of the larger-value resistor. The resistance of such devices therefore, apparently, can vary only in quantised steps depending upon the number of sub-bands present. Otherwise, the resistance is constant.